Ceramic heating elements can be used, for instance, in the case of burners, for heating a mixture to be combusted to a temperature above the ignition temperature. Such ceramic heating elements made of silicon carbide may be produced in different ways. Frequently, the silicon carbide elements are not very dense but large-pored, which leads to a low mechanical loadability.
U.S. Pat. No. 5,322,824 describes a silicon carbide ceramic composite article that is sintered without the use of pressure, and has a direct current conductivity of at least 0.05 (ohm×cm)−1, a volume density of at least 2.9 g/cm3, a bending strength of at least 100 Mpa. The composite article has conductivity properties of the p type, and includes approximately 0.5 through 6.0 weight % of aluminum and 0.1 through 2.0 weight % of boron being admixed, the aluminum being provided in an equal or greater amount of weight as boron, and which further includes at least 0.1 through approximately 6.0% free carbons and at least 90 weight % silicon carbide, the silicon carbide being present predominantly in the alpha phase. According to U.S. Pat. No. 5,322,824, the silicon carbide element has a relatively low electrical resistance value because of doping by aluminum atoms and boron atoms in the silicon carbide crystal lattice structure, specifically by a diffusion mechanism at elevated temperatures, so as to form an SiC(Al, B) semiconductor of the p type. It is further explained that aluminum doping agents have a stronger resistance value-lowering effect on silicon carbide than boron doping agents. It is also mentioned that normal grain growth occurs sooner in SiC—Al—C systems than in SiB—B—C systems, namely at a sintering temperature higher than about 2,050° C., and that this grain growth is undesirable because of deterioration of mechanical properties. According to the above reference, the silicon carbide powder that is to be used as a starting material should itself have only low traces of contaminants such as graphite, aluminum, boron or free silicon. It is also suggested in the reference that the sintering of the initial blank may be carried out in a nitrogen atmosphere, as long as the resulting electrical conductivity is not of importance.
A sintered element made of silicon carbide or boron carbide is also described in German Patent No. 42 33 626, in which preparation of a sintered body made of silicon carbide or boron carbide includes: a) suspending silicon carbide or boron carbide in an aqueous or organic medium, and producing negative or positive surface charges by setting a suitable pH value; b) admixing a sintering additive which has a surface charge of opposite polarity from that of the silicon carbide or boron carbide; c) producing an initial blank from the slip obtained; and d) sintering the initial blank to a sintered element. The following two component sintering additives C/Al, C/B, C/Al2O3 or C/B4C are used in the case of SiC.
It is mentioned that a sintered element is obtained which may be used as a structural ceramic in the high temperature range, for example, for gas turbines, combustion chambers, rotor blades and turbine wheels; in chemical apparatus construction for use with strongly corrosive media; as heat exchangers, heat conductors, fireproof materials in high temperature furnace construction; in machine construction as gliding bearing and gliding ring seals; in the grinding materials industry; and in the electrical industry for the production of varistors and rectifiers. The SiC fibers, whiskers and composites then act to improve the firmness and the fracture toughness of oxidic and nonoxidic high-performance ceramics. According to DE 42 33 626, sintering is to be carried out without the application of pressure.
German Patent Application No. 195 37 714, referencing German Patent No. 42 33 626, describes how silicon carbide materials having good electrical properties, especially good electrical conductivity, good oxidation resistance and great firmness may be produced by pressure-less sintering. It is mentioned that, beside others, aluminum is a suitable element for adding via sintering additives. It is further stated that carbon may be effective as a reducing agent during sintering and may cleanse the grain surfaces of the SiC of SiC2, which provides an increase in surface energy of the powder and the grain boundary diffusion during sintering. It is also stated that they are homogeneously distributed in the initial ceramic, so that these advantageous properties of the sintering additives can be utilized properly. It is also mentioned that, while it is true that good electrical conductivity may be achieved by doping agents such as aluminum nitride, molybdenum disilicide, phosphorus, arsenic and antimony, the sintering behavior of the ceramic is, however, influenced unfavorably by these additives, so that sufficient densification can be achieved only by pressure-supported sintering methods such as hot pressing or high-temperature isostatic pressing.
DE 195 37 714 provides that an afterglow step should be carried out in a nitrogen and/or carbon monoxide-containing atmosphere after sintering in a method for producing a conductive sintering element based on silicon carbide. This is purportedly leads to good electrical conductivity, which is especially useful for producing electric igniters. It is stated that the resistance of such an igniter may be adjusted via its geometry, and that the electric igniters described in DE 195 37 714 may be operated also at 220 V and may be dimensioned to be very small. A dumbbell-shaped electric igniter is particularly suggested, having an overall length of 60 mm and a width of 4 mm or 2 mm at a thickness of 1 mm. This should be heatable in air to a temperature of 1,300° C.
A disadvantage of the described arrangement is that the conventional SiC ceramics discussed in the reference have a negative temperature coefficient. For electrical igniters in gas or oil burners, it is usually required that a certain temperature sufficient for ignition be reached in a predefined time, and held there for a certain length of time sufficient for igniting a fuel mixture such as an air/gas and/or air/oil mixture flowing at 10 m/s past the hot electrical igniter. After the ignition, the igniter may be shut off. Because of the rapidly decreasing resistance of the conventional SiC ceramic heating elements having NTC(Negative temperature coefficient) properties, the igniter element and its mounting fixture heat up to undesirably high temperatures far above the ignition temperature. This may impair the electrical contacting and/or lead to sparkover. Attempts at integrating the NTC properties into the ceramic by fitting in a conductor having PTC properties can make hot pressing a necessary procedure, which impairs shaping.